The scientific method of determining what wavelengths of light (visible or otherwise) which are absorbed or emitted from a substance. Common examples of spectroscopy include Fourier transform infrared (FT-IR) spectroscopy, UV-Visible spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. This information gives hints about the structure of a compound, composition of a mixture, or kinetics and thermodynamics of a reaction.
Infrared spectroscopy, of which the most common and fastest is FT-IR, can reveal information about the structure of a chemical compound.
UV-Visible spectroscopy reveals information about the electronic structure of a compound.
NMR is used extensively in determining the structure of unknown compounds such as proteins and other organic compounds.